The present invention relates to a method of arranging filters of the type comprising a large number of substantially annular, thin filter elements on a radially pervious and hollow core, thereby forming slots between adjacent elements to allow passage of the liquid to be filtered, and establishing a pressure difference between the space outside the filter elements and the space within the core to permit the liquid transport through the slots.
The invention also relates to an installation for carrying the method into effect, comprising a closed vessel and at least one filter assembly projecting into the vessel and consisting of a radially pervious or perforated and hollow core closed at one end and connected at the other end with a space situated outside the vessel, and annular thin filter elements on the core and disposed between end supports of which at least one is adjustable along the core.
In filters of the type referred to the filter elements often consist of paper or paper-like material and in such filters the application pressure between the filter elements is controlled by a screw means, a spring or the like which is tightened to a certain value. The liquid to be filtered is allowed to pass radially through the filter element pile, and the purified liquid can be collected on the remote side and the impurities remain on the opposite side. For cleaning of the filter, liquid is allowed to pass in the reverse direction. In conventional filters the compression of the filter elements cannot be pushed too far since these would thereby be permanently deformed, which would gradually alter the filtering capacity.
It has been found that the differential pressure can have a negative influence on the function of the filter because the pressure in a surrounding vessel will have a compressive effect on the filter pile whereby the application between the filter elements will be so heavy that the throughflow will be entirely interrupted under unfavorable conditions. It is interesting to note that the compression becomes self-increasing since, as the slots between the filter elements decrease, the throughflow decreases and the differential pressure increases. To remedy this it has been proposed in French Patent Specification No. 1,343,366 to provide a compression-preventing fluid chamber communicating with the space inside the filter elements and adapted to prevent the pressure in the surrounding vessel from acting upon the filter elements in a compressive sense. The spring-biased end plate engaging with the free end of the filter elements is balanced with the aid of the pressure of the outgoing fluid the pressure of which acts on both sides of the end plate which is displaceable like the piston in a cylinder. Therefore the known filter is not influenced by the differential pressure in the axial sense. The degree of compression is entirely controlled by the spring which pulls the lower end plate towards the fixed end plate at the outlet opening.
The present invention is based on the knowledge that the differential pressure can act upon the compression of the filter elements, and the object of the invention is to provide a means for utilizing this knowledge in order to adjust by a simple operation the filtration degree of the filter, i.e. vary the separating capacity of the filter upwards as well as downwards and permit filtration with separation of very small particles.
The essential characteristic of the method of the invention resides in: enclosing in a closed vessel the core surrounded by a plurality of annular filter elements; connecting the interior of the core with an outlet line or the like; establishing an initial application of axial pressure on the filter elements by a spring or other means which is adapted to act upon an end support engaging an end one of the filter elements; and adjusting the application of axial pressure on the filter elements to control the size of the slots to establish the desired filtering degree by adjusting the differential pressure, i.e. the difference between the higher pressure in the closed vessel and the pressure in the interior of the core, in such a way that the plurality of filter elements will be axially compressed.
The essential characteristic of the installation of the invention resides in a plurality of filter elements consisting of a material which is recurrently compressible and substantially incapable of absorbing the filtration liquid, which plurality of filter elements surround a pervious hollow core, a spring element adapted to act upon a displaceable end support engaging the end one of the filter elements to establish an initial axial pressure on the plurality of filter elements and a vessel containing the filter assembly of filter elements surrounding the pervious core. The vessel is in the form of a pressure vessel to permit variable pressure buildup across the filter assembly so that the size of the filter slots may be varied in response to the variation in axial compression of the filter elements.